ci: cancel PR workflows on close (#4323)

.github/workflows/fuzz.yml

@@ -16,6 +16,7 @@ name: Fuzz
 
 on:
   pull_request:
+    types: [ opened, synchronize, reopened, closed ]
     paths:
       - "fuzz/**"
       - "scripts/fuzz/**"
@@ -41,7 +42,7 @@ permissions:
   contents: read
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
 
 env:
@@ -50,13 +51,21 @@ env:
   RUSTFLAGS: "--cfg tokio_unstable -C target-feature=-crt-static"
 
 jobs:
+  cancel-closed-pr-runs:
+    name: Cancel Closed PR Runs
+    if: github.event_name == 'pull_request' && github.event.action == 'closed'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Explain cancellation run
+        run: echo "PR closed; this run only cancels older runs in the same concurrency group."
+
   # ──────────────────────────────────────────────────────────────
   # Phase 1: Build all fuzz harness binaries once.
   # ──────────────────────────────────────────────────────────────
   fuzz-build:
     name: Build Fuzz Harness
     if: >
-      github.event_name == 'pull_request' ||
+      (github.event_name == 'pull_request' && github.event.action != 'closed') ||
       github.event_name == 'schedule' ||
       github.event_name == 'workflow_dispatch'
     runs-on: sm-standard-4
@@ -110,7 +119,7 @@ jobs:
     name: "Smoke / ${{ matrix.target }}"
     needs: fuzz-build
     if: >
-      github.event_name == 'pull_request' ||
+      (github.event_name == 'pull_request' && github.event.action != 'closed') ||
       (github.event_name == 'workflow_dispatch' &&
         (github.event.inputs.profile == 'smoke' || github.event.inputs.profile == 'both'))
     runs-on: sm-standard-4